Make memory_limiter and otlp_receiver return 503 status code instead of 500 on failure

[climberfree]

The problem
When the collector fails due to memory_limiter, the agent receives a 500 status code. This status code is not retryable, and the error sometimes resolves within minutes, especially after creating one more collector's instance through autoscaling.

The solution
We propose making the memory_limiter and otlp_receiver return a 503 status code instead of 500 in situations where the failure is temporary and expected to be resolved shortly.

Benefits:

• makes the situation retryable, allowing the agent to attempt a new connection after a brief delay.
• aligns with common HTTP status code semantics, indicating that the service is temporarily unavailable.

[dmitryax]

This should be already resolved and available in 0.93.0 with #8080 merged

[TylerHelmuth]

@dmitryax I think #8080 actually only fixed the problem when using grpc. If using http, we still have these line in our code:

opentelemetry-collector/receiver/otlpreceiver/otlphttp.go

Lines 105 to 109 in d78b748

	otlpResp, err := logsReceiver.Export(req.Context(), otlpReq)
	if err != nil {
	writeError(resp, enc, err, http.StatusInternalServerError)
	return
	}

With #8080 the value in err is now properly a Status with code Unavailable, but our http logic is still returning 500.